1. Field of the Invention
The present invention relates to a document handling apparatus for feeding original documents to a position for exposure and discharging said documents from said position after exposure thereof.
2. Description of the Prior Art
Such document handling apparatus is typically exemplified by a circulating automatic document feeder equipped with an image forming apparatus such as a copier.
In such known apparatus, sheet documents are stacked on a sheet document stacker tray and there is executed a cycle, for each of said documents, of separating a sheet document from said stack, feeding the thus separated document to a glass platen of said image forming apparatus where said document is to be illuminated and exposed, returning said document to said stacker tray after exposure and feeding a succeeding document to said platen glass. In this manner one or plural copies for each of the original documents can be obtained by circulating the stacked documents once or plural times, and such apparatus is extremely useful for automatic rapid copying or other processing of a large number of sheet documents.
In such known document handling apparatus it has already been proposed to simplify the structure by feeding and discharging the documents at the same time to and from a side of the platen glass. However, rapid movement of documents is extremely difficult in such a system, because they have to be moved in the forward and backward directions, and such structure requires high precision in order to prevent jamming or damage of documents.
Also, in such known document handling apparatus, the discharge of a document from the position for exposure and the feeding of a succeeding document to said position are conducted at a fixed interval, which is determined on the basis of a maximum possible size even when smaller documents are handled. For this reason the feeding of a document often requires an unnecessarily long time, thus delaying the copying operation.
Furthermore, in such known apparatus, it has been customary to detect the passing of the rear end of an original document at a determined position in the document feed path, to set said document at a position for exposure on the platen glass after a determined delay from said detection, and, after the exposure, to discharge said document simultaneously with the feeding of a succeeding document.
Consequently, if documents of plural sizes are mixed in the stack and if a larger document, for example A3 size, is exposed and to be followed by a smaller document, for example A4 size, the former may still remain on the platen glass when the succeeding one is set at the position for exposure.
Furthermore, such known apparatus is designed in such a manner that a document discharged onto the stacker tray slides thereon to a sheet feed slot spontaneously. It therefore often happens that the documents are stopped for some reason and do no reach the feed slot.
Furthermore, some of such known apparatus detect the size of document during transportation thereof by counting clock pulses generated in synchronization with the function of document transporting means, during an interval between the detections of the front and rear ends of a document with a sensor.
However, such size detecting method is hardly capable of exact size detection in the case that the apparatus contains plural document conveyors of different speeds driven by separate power sources and the document is transferred among such different conveyors. Also, such size detecting method is incapable of size determination until the document has completely passed the position of the sensor.
Furthermore, some of such known apparatus are designed to handle two-sided original documents. In such apparatus, the inversion of the document, required for making copies from such two-sided original document, is achieved by a so-called switchback operation in which a document is advanced in a direction with rollers, then stopped while being pinched by said rollers and then reversed in the opposite direction by reversing the direction of said rollers.
However, in such switch-back operation, the reversing of the transport direction while the document is pinched between the rollers tends to apply an uneven advancing force to the document, thus resulting in problems such as skewed advancement.
Also, such apparatus has two operation modes; i.e., a first mode in which the original document has to be precisely stopped at the position for exposure on the platen glass, and a second mode in which the document need not be stopped at a precise position on the platen glass as it is used as a part of a switch-back path for inverting the document. However, the conventional apparatus has employed a control method for precisely stopping the document in both modes, thus requiring a lower transport speed and a more complicated control process than would otherwise be needed.